In recent years, in an optical pickup apparatus, a wavelength of a laser light source which is employed as a light source for reproducing information recorded on an optical disc and for recording information on an optical disc, is becoming shorter. For example, a laser light source having 400-420 nm wavelength, such as a blue-violet semiconductor laser and a blue-SHG laser which converts wavelength of an infrared semiconductor laser utilizing a second harmonic wave, is reaching the stage of practical application. A use of these blue-violet laser light sources enables to record information of about 15 GB on an optical disc with a diameter of 12 cm, when an objective lens with a numerical aperture (NA) which is the same as a DVD (Digital Versatile Disc) is used. When NA is increased up to 0.85, information of about 25 GB can be recorded onto an optical disc with a diameter of 12 cm. In this specification, an optical disc and an optical-magnetic disc which employ a blue-violet laser light source are referred as “high density optical discs” as a general name.
Hereupon, in a high density optical disc using an objective lens with NA of 0.85, coma caused due to an inclination (skew) of the optical disc becomes larger. Therefore, some high density optical discs using the objective lens with NA of 0.85, are designed so that its protective layer has thinner thickness (which is 0.1 mm, while that of a DVD is 0.6 mm) than that of a DVD, to reduce the amount of coma due to the skew. On the other hand, it is hard to say that a value of an optical disc player/recorder (optical information recording reproducing apparatus) as a product is not enough under just a condition that it can record and/or reproduce information for the above high density optical discs properly. Taking account of a fact that, at present, DVDs and CDs (Compact Disc) onto which various kinds of information have been recorded, are on the market, it is not enough that information can be recorded/reproduced only for the above high density optical disc properly. For example, realization of recording and/or reproducing information appropriately also for DVDs and CDs which are owned by users, leads to enhancement of its commercial value as an optical disc player/recorder for high density optical discs. From these backgrounds, an optical pickup apparatus installed in an optical disc player/recorder for high density optical discs is required to be capable of appropriately recording and/or reproducing information not only for the high density optical discs but also for DVDs and CDs.
As a method capable to record and/or reproduce information adequately to anyone of high density optical discs and DVDs and further to CDs, with maintaining compatibility, there can be considered a method to selectively switch an optical system for high density optical discs and an optical system for DVDs and CDs, corresponding to the recording density of an optical disc on which information is recorded and/or reproduced. However, it is disadvantageous for the size-reduction and increases the cost, because it requires a plurality of optical systems.
Accordingly, in order to simplify the structure of an optical pickup apparatus and to intend the reduction of cost, it is preferable to make an optical system for the high density optical discs and an optical system for DVDs and CDs into a common optical system, and to reduce the number of optical parts constructing the optical pickup apparatus as much as possible, even in the optical pickup apparatus with compatibility. Then, providing the common objective lens which is arranged with facing an optical disc, is most advantageous for the simplification of the construction and cost reduction of the optical pickup apparatus. Here, in order to obtain an objective lens which can be commonly used for plural kinds of optical discs for which different recording/reproducing wavelengths are used, it is required that there is formed an optical path difference providing structure having a wavelength dependency in spherical aberration, on the objective optical system, to reduce spherical aberrations caused by a difference in wavelength and a difference in thickness of protective layers.
Patent Literature 1 discloses an optical pickup apparatus equipped with an objective lens which includes an optical path difference providing structure of a step type with four steps and can be used compatibly for high density optical discs and conventional optical discs.